1. Field of the Invention
The present invention relates to a method for mounting electronic device elements, and more particularly, the present invention relates to a method for mounting surface acoustic wave devices on mounting substrates by applying an ultrasonic wave thereto and pressing surface acoustic wave devices on the mounting substrates.
2. Description of the Related Art
Corresponding to recent miniaturized and thin electronic components, a flip chip bonding method has been developed as a method of mounting electronic device elements on substrates. The flip chip bonding method is a mounting method that positions a functional surface of an electronic device element relative to a substrate such that the functional surface opposes the substrate, then affixes the electronic device element thereon. The method is used to electrically and mechanically connect metal bumps formed on electrodes of an electronic device element and electrode patterns on the substrate. Further, the method is used to connect metal bumps located on electrode patterns provided on a substrate to electrodes of an electronic device element. Flip chip bonding methods currently in use include a method that applies an ultrasonic wave at the time of connection, and a method that simultaneously applies an ultrasonic wave and heat at the time of connection. These methods are used to increase the strength of the connections between the metal bumps and the electrode patterns provided on the mounting substrate.
In the flip chip bonding method that uses only the ultrasonic wave, increasing the ultrasonic wave power that is applied increases the strength of the connections between the metal bumps and the electrode patterns provided on the mounting substrate. However, when an ultrasonic wave power is excessively increased, a great deal of stress is exerted on the electrodes on the element and the substrate that defines the element, thereby causing cracks in the electrodes and the substrate.
Japanese Unexamined Patent Application Publication No. 8-330880 discloses a method as shown in FIGS. 14A and 14B that causes the above-mentioned problems. More specifically, as shown in FIG. 14A, although the direction of a load exerted by a bonding tool 67 is perpendicular to an electrode surface of an electronic device element 61, the ultrasonic-wave-oscillation direction is horizontal. Therefore, a resultant force of the load and the ultrasonic wave, as shown in FIG. 14B, is exerted onto the electronic device element 61. As a result, the level of a side portion of the electronic device element 61 is significantly lower than the level of the central portion. Accordingly, energy is concentrated on a metal bump 64 positioned on a side portion of the electronic device element, and the metal bump 64 collapses more than a metal bump 65 located in the central portion, thereby causing cracks in the metal bump 64 and an electrode pad 62.
To solve the aforementioned problems, Japanese Unexamined Patent Application Publication No. 8-330880 suggests that dummy bumps be provided on the electronic device element. Specifically, as shown in FIG. 15, dummy pads 73 and dummy bumps 74 are provided farther away from the central portion in the ultrasonic-wave-oscillation direction than electrode pads 72 and metal bumps 75 provided on an electronic device element 71, thereby allowing stress to be concentrated on the dummy pads 73 and the dummy bumps 74. This prevents the cracks from forming on the electrode pads 72 and the metal bumps 75 that provide the necessary electrical conduction to a mounting substrate provided on the element 71.
However, the method wherein the dummy bumps are provided for preventing the aforementioned cracks from forming is plagued by the following problems. According to the above-described method, since the dummy pads and the dummy bumps must be additionally provided, the size of the substrate of the electronic device element must be increased. This causes a problem in that the size of the overall electronic component is increased. Further, when cracks occur in the dummy pads, pieces of pads may be separated from the dummy pads. If the pieces adhere to the surface of the electronic device element, electrical characteristics of the electronic component and the reliability thereof deteriorate.
To overcome the problems described above, preferred embodiments of the present invention provide a method of mounting electronic device elements for producing small electronic components that have a high reliability in coupling between an electronic device element and a package and excellent electrical characteristics.
A preferred embodiment of the present invention includes the steps of providing an electronic device element having metal bumps provided on a surface thereof, providing a bonding tool having a pressing surface, providing a mounting substrate, keeping the pressing surface of the bonding tool in contact with a reverse surface of the electronic device element, and applying an ultrasonic wave to the bonding tool, thereby mounting the electronic device element on the mounting substrate, wherein the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is greater than approximately half the maximum length of the reverse surface of the electronic device element in the ultrasonic-wave-oscillation direction.
In this way, by increasing the length of the pressing surface of the boding tool in the ultrasonic-wave direction, the length of a surface wave acoustic wave device in the oscillation direction in contact with the bonding tool is increased. Thus, at the time of packaging, even when a resultant force of the load and the ultrasonic wave is momentarily exerted in a tilted orientation, the surface acoustic wave device is not tilted with respect to the horizontal direction of the mounting substrate nor does stress concentrate on metal bumps in specific positions in the electronic device element. Therefore, occurrence of cracks in the metal bumps and electrode pads is greatly reduced, thereby providing stable and reliable coupling with excellent electrical characteristics between the electronic device element and the mounting substrate.
Another preferred embodiment of the present invention provides that the shape of the pressing surface of the bonding tool is substantially the same as the shape of the reverse surface of the electronic device element. In this way, by selecting the shape of the pressing surface of the bonding tool to be substantially the same as the shape of the reverse surface of the electronic device element, the length of the electronic device element in contact with the bonding tool is further increased in directions other than the ultrasonic-wave-oscillation direction. That is, the element has a large area in contact with the boding tool such that stable contact is provided. As a result, stress is not concentrated on metal bumps in specific positions in the element. As a result, occurrence of cracks on metal bumps and electrode pads is greatly reduced, thereby further improving the coupling of the electronic device element to the substrate.
Further, in another preferred embodiment of the present invention, the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is substantially equal to or less than the maximum length of the reverse surface of the electronic device element in the ultrasonic-wave-oscillation direction.
By arranging the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction to be substantially equal to or less than the maximum length of the reverse surface of the electronic device element in the ultrasonic-wave-oscillation direction, when the electronic device element is packaged in the package, interference with sidewalls of the package when the ultrasonic wave is applied is prevented without increasing the size of the package as compared to the size of the electronic device element. Therefore, miniaturization of electronic components is achieved.
By arranging the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is considerably less than the length of the reverse face of the electronic device element in the ultrasonic-wave-oscillation direction, even when the pressing surface of the bonding tool deviates from the reverse surface of the electronic device element, there is a small distance between the periphery of the pressing surface of the bonding tool and the periphery of the reverse surface of the element. Therefore, the interference caused by the bonding tool and the package is prevented.
Another preferred embodiment of the present invention provides that the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is approximately 0.8 times the maximum length of the reverse surface of the electronic device element in the ultrasonic-wave-oscillation direction.
The length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is preferably such that the electronic device element does not substantially tilt with respect to the horizontal direction of the mounting substrate. In addition, the length is also preferably arranged to prevent between the sidewall of the package and the bonding tool even when the pressing surface of the bonding tool somewhat deviates from the reverse surface of the electronic device element. To satisfy these conditions, the maximum length of the pressing surface of the bonding tool in the ultrasonic-wave-oscillation direction is set to be approximately 0.8 times the maximum length of the reverse surface of the electronic device element in the ultrasonic-wave-oscillation direction.
Another preferred embodiment of the present invention provides that a vacuum opening for securing the electronic device element is provided on the pressing surface of the aforementioned bonding tool. The vacuum opening is positioned at a portion of a reverse surface of the electronic device element where bumps are not located, such that very stable and reliable mounting of the electronic device element on the mounting substrate is achieved.
Thus, by providing the bonding tool with the vacuum opening to enable the bonding tool to pick up the electronic device element and to position the electronic device on the mounting substrate, the ultrasonic wave power which is applied to portions of the electronic device element positioned directly under the vacuuming opening is reduced with respect to a power that is exerted on a portion of the electronic device element which is in direct contact with the bonding tool. Therefore, by providing the vacuuming opening in positions on the reverse surface of a portion where the metal bump of the electronic device element is not located, the ultrasonic wave power which is exerted on individual metal bumps is uniform. In this case, stress is not concentrated on metal bumps on specific positions. Therefore, occurrence of cracks in the metal bumps and a metal bump is greatly reduced, thereby providing stable and very reliable coupling of the electronic device element to the substrate.
Another preferred embodiment of the present invention provides that the aforementioned electronic device element is a surface acoustic wave device. In a surface acoustic wave device, since a space must be provided on the side of a functional surface of the device to allow surface waves to be generated and propagated freely, the device cannot be immobilized by providing an adhesive between the device and a package. Therefore, metal bumps must be used to electrically and mechanically couple the device and the package together. The metal bumps are arranged to provide a high coupling strength and reliability.
Another preferred embodiment of the present invention provides a method of manufacturing a surface acoustic wave unit including using the mounting method of the above preferred embodiments to incorporate a surface acoustic wave device into a package, and closing the package by fixing a cap to the package. The method of mounting the electronic device elements according to preferred embodiments of the present invention can be used to package the surface acoustic wave device. That is, the mounting method may be applied to methods of manufacturing of surface acoustic wave devices.
Other features, elements, characteristics and advantages of the present invention will become more apparent with reference to preferred embodiments thereof which are described in detail below with reference to the attached drawings.